Image intensifier tubes are widely used to magnify low light signals. Image intensifiers based on micro-channel plate (MCP) and proximity focus concept can provide high gain due to MCP magnification, low distortion, and uniform resolution across an entire field of view. However, MCP based image intensifiers tend to have relatively bad resolution for many critical applications. In addition, MCP may block as much as 40% of the photoelectrons right after the photocathode. Thus, detective quantum efficiency for MCP based image intensifiers is usually low.
To achieve higher detective quantum efficiency, intensifier designs based on electrostatic focusing lens and combined magnetic-electrostatic focusing tube may be utilized. Pure electrostatic image intensifiers usually have high distortion and field-curve aberration. Some electrostatic image intensifiers have either curved photocathode plane or curved scintillating screen (e.g. phosphor screen) plane. However, upstream illumination optics and downstream collection light optics usually have flat image and object field. As a result, electrostatic image intensifiers are not suitable for applications requiring both high spatial resolution and low distortion.
Conventional magnetically focused image intensifier tube design has been discussed in detail in publications, such as Electro-Optics Handbook, R. Waynant and M. Ediger, McGraw-Hill (1994). Electron optics has been discussed in detail in IRE transactions on Nuclear Science, volume 9, issue 2, pages 91-93. Conventional electron optics for magnetically focused intensifier is based on the concept of uniform electric accelerating field {right arrow over (E)} and homogeneous magnetic focusing field {right arrow over (B)} along the tube axis. When photoelectrons are emitted from a photocathode in response to incident illumination, their initial velocity has a transverse component. Transverse velocity is perpendicular to the magnetic field lines. As a result, photoelectrons with non-zero transverse velocity will rotate along the magnetic field lines while being accelerated from the photocathode towards a scintillating screen disposed at an opposite end of the tube. The focusing condition is that photoelectrons make a full integer number of turns. Depending on {right arrow over (B)} field strength, more than one focus node may exist inside the tube. The time for photoelectrons to make one full turn in magnetic field {right arrow over (B)} may be determined by the following equation:
      T    =                  2        ⁢                  π          ⁢                                          ·                      m            e                                      e        ·        B              ,where e is electron charge, me is electron mass and B is the magnetic field strength.
The focusing condition is satisfied once electrons travel from the photocathode to the scintillating screen in time interval nT, where n is an integer. Electron travel time is determined by electric accelerating field strength {right arrow over (E)}. The focusing power is substantially the same everywhere when there are uniform {right arrow over (E)} and {right arrow over (B)} fields. To create uniform {right arrow over (B)} field, a magnetic solenoid disposed outside to the intensifier tube may need to be at least three times the length of the tube. This is in order to generate relatively uniform magnetic field across the distance occupied by the intensifier tube. Due to design constraints, shorter magnetic solenoids are typically preferred. However, magnetic field is typically not uniform with a shorter magnetic solenoid. Degradation of resolution due to non-uniform magnetic field is mentioned in IRE Transaction on Nuclear Science, volume 9, issue 4, pages 55-60.
The {right arrow over (B)} field lines generated by a short magnetic solenoid are usually divergent around the photocathode and the scintillating screen. Off-axis photoelectrons may be bent towards the tube center right after photocathode and then bent outwards. As a result, the distance traveled by off-axis photoelectrons may be longer than that of on-axis photoelectrons. The off-axis photoelectrons will, therefore, be focused before they arrive at the scintillating screen. This kind of focusing error is known as field curvature aberration. If soft ion pole pieces are used to shield outer electromagnetic interference, the magnetic field strength may become stronger at off-axis locations compared with the magnetic field strength at the center of the tube. Stronger {right arrow over (B)} field at off axis points can further increase the field curvature aberration. High field curvature aberration results in non-uniform resolution from the center to the edge of the field of view.
Lifetime of magnetic focus image intensifiers is also currently limited by damage from ions accelerated toward the photocathode, as discussed in U.S. Pub. No. 2007/0051879 A1. Photoelectrons will deposit accumulated energy into the scintillating screen and excite cathodoluminescence emission. In the meantime, secondary and backscattering electrons may be knocked out of the scintillating screen surface. The low energy secondary and backscattering electrons have high electron-impact ionization cross section and may create positive ions around the scintillating screen area. The positively charged ions are then accelerated backwards through the tube towards the photocathode. Back-bombardment from ions can cause serious damage to the photocathode and reduce quantum efficiency.
The foregoing deficiencies hinder utilization of magnetic focus image intensifiers in many applications. New designs to overcome one or more of the foregoing deficiencies will be appreciated by those skilled in the art.